The invention relates to a weighing pan carrier for a balance with a weighing pan and a weighing cell, the latter having a load-receiving portion and a stationary portion. The weighing pan carrier is attached to the load-receiving portion of the weighing cell, and an overload protection device is in releasable engagement with the weighing pan carrier.
The overload protection device in a balance has the function of uncoupling the load-receiving device, in particular the weighing pan, from the weighing cell when a maximally permissible load is exceeded, where the load causes a substantially vertical force to act on the weighing pan. As a result, the weighing cell is protected from the effect of excessive, primarily vertically directed forces, which can either be of a static nature, caused by a weight that exceeds the maximally permissible load, or of a dynamic nature, caused for example by a load being applied too abruptly.
An overload protection device that meets the foregoing description is disclosed in U.S. Pat. No. 6,354,159 B2. A load-receiving portion of a force-measuring device is described, where the load-receiving portion has two parts held in mutual engagement by means of a pre-tensioned elastic element. When a load in excess of the defined maximum load is introduced into the force-measuring device, the second of the two parts is displaced relative to the first part against the pro-tension of the elastic element until the accelerating movement of the second part is held up by an end stop. The foregoing concept, where the first part which serves to introduce the force is uncoupled from the second part which serves to transmit the force to the measuring cell, thus prevents damage to the force-measuring cell in case of an overload.
A force-measuring device which is disclosed in U.S. Pat. No. 6,232,567 B1, in particular a balance, has a weighing pan carrier coupled to a seating cone for the weighing pan, using a force-biased connection by means of a pre-tensioned helix spring. The helix spring is guided by the cylindrical outside surface of a tubular part that is solidly connected to and stands up vertically from the weighing pan carrier, while a bolt connected to the seating cone is slidably guided inside the tubular part. The bolt and the tubular part protrude from the underside of the weighing pan carrier into a recess of the stationary portion of the weighing cell. As long as the load on the weighing pan is within the specified range, the lower ends of the bolt and tubular part remain connected to each other. When an excessive force is directed vertically at the load-receiving portion, for example as a result of an excessive load being put on the weighing pan, the connection releases itself. The bolt moves downward until it comes to rest on a stationary end stop in the recess.
Disclosed in DE 94 06 898 U1 is a device for a balance, which serves on the one hand as an overload protection device against excessive forces directed vertically at the balance pan and on the other hand protects the load-receiving suspension against laterally directed forces causing the load receiver to swivel or tilt, including forces that are, e.g., directed at an oblique angle from above or horizontally from the side, particularly as a result of an eccentrically placed weighing load.
A balance with a single-point support for the weighing pan is disclosed in U.S. Pat. No. 4,574,898, which provides protection against tilting of the weighing pan due to eccentrically placed loads by means of rest stops that are attached to the base of the balance below the borders of the weighing pan, e.g., all around the weighing pan.
With the trend towards more frequent quality checks during the production process, there is an increasing demand for the use of balances. Balances of modular configuration, so-called weighing modules, are particularly well suited for integration in automated production- and quality control systems. Applications where such integrated weighing modules are preferred are found in the production and checking of small, expensive parts, for example in filling and packaging machines for tablets in the pharmaceutical industry, or in the quality control of ball bearings. It is often necessary to attach an application-specific superstructure to the weighing pan or to a special load-receiving platform. Because a high degree of reproducibility is required in the weighing results, the load-receiving platform preferably has a solid connection to the load-introducing part of the weighing cell, in particular a weighing pan carrier. However, when using a solid connection between the load-receiving part and the weighing cell, the weighing cell needs to be protected against damage from torques that are caused by forces in transverse directions relative to the direction of the weighing load, for example forces that are directed tangentially at the weighing pan. Forces of this kind occur in particular in cases where a superstructure element is solidly screwed onto the load-receiving device, specifically where the weighing pan is screwed onto the weighing pan carrier.
Devices for the measurement of torques are known to have a so-called overload protection feature, i.e., a safety device against excessive amounts of torque being applied to the measuring device. An example of a torque overload protection device is disclosed for example in EP 0279 182 A1. A measuring shaft and a torque-introducing shaft are each equipped with one half of a clutch, where the clutch halves have teeth which during normal operation are pushed together into mutual engagement by means of a pre-tensioned spring. When an excessive amount of torque is applied, the two clutch halves release their mutual engagement against the force of the spring. This brings the backside of one of the clutch halves into contact with a friction surface of a surrounding stationary housing. The gap between the friction surface and the clutch half is significantly smaller than the height of the clutch teeth. This has the result that a part of the excessive torque is taken up by the housing so that the measuring device is partially relieved.
To protect a balance against torques due to forces acting in a direction perpendicular to the direction of the weighing load, one needs to find a solution that is specifically adapted to the weighing function, i.e., to the measurement of a load acting in the direction of the gravity force. A solution meeting this requirement is not known for the state-of-the-art balances of the foregoing description.